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Chapter 23

Chapter 23. Introduction to General, Organic, and Biochemistry, 10e John Wiley & Sons, Inc Morris Hein, Scott Pattison, and Susan Arena. Aldehydes and Ketones. Ketones are common solvents for quick-drying paints.

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Chapter 23

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  1. Chapter 23 Introduction to General, Organic, and Biochemistry, 10e John Wiley & Sons, Inc Morris Hein, Scott Pattison, and Susan Arena Aldehydes and Ketones Ketones are common solvents for quick-drying paints.

  2. Aldehydes (i.e. RCHO)and ketones (i.e. R2CO)are compounds with a chemical structure thatcontains the carbonyl (C=O) functional group as shown in these structures (R– is an alkyl group and Ar– is an aromatic group). Structures of Aldehydes and Ketones 2

  3. Structures of Aldehydes and Ketones The aldehyde group is often written as CHO. For example, The ketone group is often written as CO. For example, 3

  4. Naming Aldehydes and Ketones IUPAC Rules for Naming Aldehydes 1. Name the longest continuous carbon chain containing the CHO group. 2. The CHO carbon is the #1 carbon atom. Notice that the aldehyde group is not given a number in the name. 4

  5. Naming Aldehydes and Ketones 3. Drop an –e from the corresponding alkane parent name and add the suffix –al. 4. Number and name groups attached to the longest carbon chain. 5

  6. Naming Aldehydes and Ketones 6

  7. Naming Aldehydes and Ketones The IUPAC names and commonly used names of some aldehydes are on Table 23.1. 7

  8. Naming Aldehydes and Ketones The simplest aromatic aldehyde is called benzaldehyde. 8

  9. Naming Aldehydes and Ketones Dialdehydes are compounds that contain two aldehyde groups. They are named by adding the suffix –dial to the corresponding hydrocarbon name. For example a molecule with four carbons and two aldehyde groups is called butanedial. 9

  10. Naming Aldehydes and Ketones IUPAC Rules for Naming Ketones 1.Name the longest continuous carbon chain containing the C=O group. 2. Drop the –e from the corresponding alkane parent name and add the suffix –one. 10

  11. Naming Aldehydes and Ketones 3. Carbon chains with four or more carbon atoms are numbered so the carbonyl (C=O) carbon is given the lowest possible number. 4. Attached groups are named and numbered as is done when naming aldehydes. 11

  12. Naming Aldehydes and Ketones An alternative non-IUPAC method that is often used to name simple ketones lists the names of the alkyl or aromatic groups attached to the carbonyl carbon together with the word ketone. Thus, butanone is also called methyl ethyl ketone. 12

  13. Naming Aldehydes and Ketones Aromatic ketones are named in a fashion similar to that of aliphatic ketones and often have special names as well. 13

  14. Your Turn! Write the structure of butyl ethyl ketone. 14

  15. Your Turn! Give names for the following compounds. 15

  16. Your Turn! Write the structure of cyclohexanone. 16

  17. Bonding and Physical Properties All aldehydes and ketones have carbonyl groups. The carbonyl carbon in a carbonyl group is sp2-hybridized with one pi (π) bond and three sigma (σ) bonds. The pi (π) bond of the carbonyl group is reactive and undergoes addition reactions. The carbonyl group is polarized which causes aldehydes and ketones to be reactive. 17

  18. Bonding and Physical Properties Unlike alcohols, aldehydes and ketones cannot hydrogen-bond to themselves, because no hydrogen atom is attached to the oxygen atom of the carbonyl group. 18

  19. Bonding and Physical Properties Therefore aldehydes and ketones have lower boiling points than alcohols of comparable molar mass as seen on Table 23.2 on the next slide . . . 19

  20. Bonding and Physical Properties 20

  21. Bonding and Physical Properties Low-molar-mass aldehydes and ketones are soluble in water. The lower-molar-mass aldehydes have a penetrating, disagreeable odor. As the molar mass increases, the odor of both aldehydes and ketones—especially the aromatic ones—becomes more fragrant. Some aldehydes and ketons are used in flavorings and perfumes . . . 21

  22. Bonding and Physical Properties 22

  23. Bonding and Physical Properties Other naturally-occurring aldehydes and ketones are shown below. 23

  24. Your Turn! Rank these molecules based on boiling point from lowest to highest. • 3-hexanol (molar mass = 102 g/mol) • 3-hexanone (molar mass = 100 g/mol) • heptane (molar mass = 100 g/mol) 24

  25. Chemical Properties of Aldehydes and Ketones The carbonyl functional group is the reactive site for aldehydes and ketones. These compounds undergo three broad classes of reactions. Reaction TypeAldehydes or Ketones 25

  26. Chemical Properties of Aldehydes and Ketones Oxidation Aldehydes (RCHO) are oxidized with dichromate to carboxylic acids (RCOOH). Here acetaldehyde oxidizes to acetic acid. 26

  27. Chemical Properties of Aldehydes and Ketones Three well-known identification tests for aldehydes are based on the fact that aldehydes are much easier to oxidize than ketones. These tests are Tollens, Fehling, and Benedict tests . . . 27

  28. Chemical Properties of Aldehydes and Ketones Aldehydes can be identified using the Tollens test(i.e. the silver-mirror test). 28

  29. The aldehyde group (RCHO) is oxidized to a carboxylic acid by Cu2+ ions in both the Fehling and Benedict tests. The tests are very similar except the Fehling test uses tartaric acid to complex Cu2+ while the Benedict test uses citric acid. Chemical Properties of Aldehydes and Ketones 29

  30. The positive test results for these tests are shown here. Chemical Properties of Aldehydes and Ketones Test Positive Test Result for Aldehydes All three tests are used to distinguish aldehydes from ketones. These tests give a positive test result for aldehydes and a negative test result for ketones. 30

  31. Would you expect a positive or negative test in the following? Your Turn! 1. Propanone in the Tollens test 2. Pentanal in the Fehling test 31

  32. Reduction Aldehydes and ketones are easily reduced to alcohols using LiAlH4, NaBH4 , or H2/Ni . Aldehydes yield primary alcohols and ketones yield secondary alcohols. Chemical Properties of Aldehydes and Ketones 32

  33. Chemical Properties of Aldehydes and Ketones Oxidation and reduction of aldehydes and ketones is important in biochemistry. Pyruvic acid is reduced to lactic acid in muscle cells and then lactic acid is oxidized back to pyruvic acid in the liver. 33

  34. This reaction sequence is important because the body uses this sequence as an additional pathway to regenerate glucose. Chemical Properties of Aldehydes and Ketones 34

  35. Chemical Properties of Aldehydes and Ketones Addition Addition reactions of aldehydes (RCHO) and ketones (R2CO). Reactants Product 35

  36. Chemical Properties of Aldehydes and Ketones Hemiacetals, Hemiketals, Acetals and Ketals Compounds derived from aldehydes and ketones that contain an alkoxy (ether) and a hydroxy group on the same carbon atom are known as hemiacetalsand hemiketals. 36

  37. Chemical Properties of Aldehydes and Ketones Compounds that have two alkoxy (ether) groups on the same carbon atom are known as acetalsand ketals. 37

  38. Chemical Properties of Aldehydes and Ketones Addition of Alcohols Aldehydes react with alcohols and a trace of acid to yield hemiacetals. 38

  39. Chemical Properties of Aldehydes and Ketones In the presence of excess alcohol and a strong acid such as dry HCl, aldehydes or hemiacetals react with a second molecule of the alcohol to yield an acetal. 39

  40. Chemical Properties of Aldehydes and Ketones Cyclic hemiacetals or hemiketals can form when the alcohol and the carbonyl group exist within the same molecule. 40

  41. Write the structures and IUPAC names for the hemiacetal and acetal formed from the reaction of butanal and ethanol. Your Turn! 41

  42. Your Turn! The conversion of 2-pentanone to 2-pentanol is a(n): • Hydration • Dehydration • Hydrolysis • Oxidation • Reduction 42

  43. Common Aldehydes and Ketones Formaldehyde (methanal) is made from methanol by reaction with oxygen in the presence of a silver or copper catalyst. Formaldehyde is marketed as a 37% aqueous solution called formalin. The largest use of formaldehyde is in the manufacture of polymers. About 1.33 x 109 kg of formaldehyde are manufactured annually in the United States. 43

  44. Common Aldehydes and Ketones Acetaldehyde (ethanal) has a general narcotic action and, in large doses, may cause respiratory paralysis. Its principal use is as an intermediate in the manufacture of other chemicals, such as acetic acid and 1-butanol. Acetic acid is made by air oxidation of acetaldehyde. 44

  45. Common Aldehydes and Ketones Acetone (propanone) is widely used as an organic solvent in the manufacture of drugs, chemicals, and explosives; for the removal of paints, varnishes, and fingernail polish; and as a solvent in the plastics industry. It is manufactured by the oxidation of 2-propanol. 45

  46. Common Aldehydes and Ketones Methyl ethyl ketone (MEK) is also widely used as a solvent, especially for lacquers. MEK is made by oxidation of 2-butanol. 46

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